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I've a doubt I can't seem to get cleared.

Carbon has 4 electrons in its last energy level right? But what do I call the bonds that it makes? The same thing goes with my main question... when do I say there are three/two single bonds and when do I call it a double/ triple bond?

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    $\begingroup$ When you see a double bond between two atoms, say "double bond". When you see two bonds from one atom to two different other atoms, say "two single bonds". $\endgroup$ – Ivan Neretin Oct 21 '16 at 10:45
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    $\begingroup$ To all close voters: I think the question is perfectly clear. If you are confused about the OP's question then comment to request clarification. $\endgroup$ – bon Oct 21 '16 at 11:51
  • $\begingroup$ Echoing what bon said. This question is NOT unclear. Voting to leave open. $\endgroup$ – M.A.R. ಠ_ಠ Oct 21 '16 at 12:47
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It boils down to orbital hybridization, which is related to the amount of negative charge centers an atom has. A lone carbon atom has 4 electrons available for bonding, so if it bonds to 4 hydrogen atoms it has 4 negative charge centers around it. In this case, because it has 4 negative charge centers we say the carbon atom is $sp^3$ hybridized, meaning that the s orbital and the three p orbitals have mixed to create 4 $sp^3$ hybrid orbitals in a tetrahedral shape around the carbon atom. These hybrid orbitals bond with the hydrogen by overlapping with its orbital/electron cloud, which is known as a sigma bond, or $\sigma$ bond. $\sigma$ bonds are any bonds formed by the overlapping of hybridized orbitals, and their electron density is on the bond axis.

However, in a double bond, such as $C_2H_4$, there are only 3 negative charge centers around each carbon atom, and thus the orbitals are only $sp^2$ hybridized, which means that there is still an unhybridized p orbital around each carbon atom. While the hybridized orbitals overlap and form a sigma bond, the unhybridized p orbitals also form a bond (the second bond in the double bond). Because these bonds are formed by two unhybridized orbitals, they are known as pi bonds, or $\pi$ bonds.

Essentially, every covalent bonding between two atoms has one sigma bond, but if it's a double bond it also has a pi bond, and if it's a triple bond it has two pi bonds along with the sigma bond.

The non-theoretical definition would be if there are two or three bonds (one sigma and one or two pi bonds) between two atoms, then it is a double or triple bond respectively. If an atom is bonded to two other atoms by a sigma bond for each, then it has two single bonds.

c2h4 hybridization

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